1. Field of the Invention
This invention relates to a chill roll for use in preparing a permanent magnet material of a R--Fe--B system containing R (wherein R represents a rare earth element inclusive of Y, hereinafter), Fe or Fe and Co, and B by a rapid quenching process, a method for preparing a permanent magnet material using the same chill roll, a permanent magnet material, and a permanent magnet material powder
2. Prior Art
As high performance rare earth magnets, powder metallurgical Sm--Co series magnets having an energy product of 32 MGOe have been commercially produced in a mass scale. These magnets, however, undesirably use expensive raw materials, Sm and Co. Among the rare earth elements, those elements having a relatively low atomic weight, for example, cerium, praseodymium and neodymium are available in plenty and less expensive compared to samarium. Further Fe is less expensive than Co. Thus R--Fe--B series magnets such as Nd--Fe--B magnets were recently developed as seen from Japanese Patent Application Kokai No. 9852/1985 disclosing rapidly quenched ones.
The rapid quenching process is to inject a metal melt against a surface of a quenching medium for quenching the melt, thereby obtaining the metal in a thin ribbon, thin fragment or powder form. The process is classified into a single roll, twin roll, and disk process depending on the type of quenching medium. Among these rapid quenching processes, the single roll process uses a single chill roll as the quenching medium. An alloy melt is injected through a nozzle against the circumference of the chill roll rotating relative to the nozzle for contacting the melt with the chill roll circumference, thereby quenching the melt from one directions for obtaining a quenched alloy typically in ribbon form. The cooling rate of the alloy is generally controlled by the circumferential speed of the chill roll. The single roll process is widely used because of a reduced number of mechanically controlled components, stable operation, economy, and ease of maintenance. The twin roll process uses a pair of chill rolls between which an alloy melt is interposed for quenching the melt from two opposite directions.